The present invention relates to the field of implantable medical devices. More particularly, the present invention relates to cardiac pacing systems that are capable of distinguishing captured beats from non-captured beats in a pacing system, especially a pacing system for a mammalian heart.
Implantable pulse generators (or IPGs) are well known in the prior art. Most IPGs include sense amplifier circuitry for detecting intrinsic cardiac electrical activity so that the IPGs may be inhibited from generating unnecessary stimulating pulses when a heart is functioning properly.
Dual-chamber cardiac pacemakers typically include separate sense amplifiers for atrial and ventricular sensing. The sense amplifiers detect the presence of signals intrinsic to the heart. Two forms of these intrinsic signals occur naturally: P-waves in the atrium and R-waves in the ventricle. Upon detecting an intrinsic signal, the sense amplifier circuitry generates a digital signal to be output to other components. These components are used to inhibit the delivery of a pacing pulse to either or both chambers if P-waves and or R-waves are occurring properly in the appropriate chamber.
It is desirable to measure reliably the response of the heart evoked by an electrical stimulation pulse (e.g. a captured beat). The measurement of the evoked response permits the determination of a patient""s stimulation threshold, e.g., the minimum energy a stimulating pulse must contain for a cardiac response to be evoked. Once a patient""s stimulation threshold is determined, the energy content of stimulating pulses may be adjusted to avoid delivering pulses with unnecessarily high energy content. Minimizing the energy content of stimulating pulses reduces power consumption, a key concern in the context of battery-powered implantable devices. Minimizing the energy content of stimulating pulses may also reduce possible side effects such as inadvertent stimulation of the diaphragm.
Detection and measurement of the response evoked by a stimulating pulse may also be useful in controlling a pacemaker""s pacing rate, in ascertaining the physiological effect of drugs or in diagnosing abnormal cardiac conditions.
There are typically two electrode-tissue interfaces in a pacing circuit: one for the tip electrode, and one for the ring (or case) electrode. Generation and delivery of an electrical heart stimulating pulse (pacing pulse) from either electrode gives rise to the storage of charge at the electrode-tissue interface. A residual post-pace polarization signal (also called stimulation polarization artifacts, xe2x80x9cafter-potentials,xe2x80x9d or polarization signals) may be generated as this stored energy dissipates after the pacing event. The tip electrode is the primary after-potential storage element in comparison to the ring electrode.
Generation and delivery of a pacing pulse may also evoke a response signal in the cardiac tissue. The evoked response signal is generally the desired result of a pacing pulse while the polarization signal is simply a residual artifact of the pacing pulse. Typically, the polarization signal may even be considered an unwanted product of the stimulus pulse. However it is difficult for conventional pacemakers to differentiate between the two. Additionally, post-pace polarization signals typically have amplitudes higher than the evoked response signal; the evoked response may thus be superimposed on the polarization signal. Consequently, it becomes difficult, if not impossible, to detect an evoked response signal using a conventional pacemaker or PCD sense amplifier employing conventional frequency filtering techniques. Polarization signals typically also have larger amplitudes than those signals intrinsic to the heart. Thus, polarization signals may also interfere with the detection and analysis of an evoked response signal in comparison to the intrinsic signals of the heart.
To overcome this difficulty, most pacemakers are set to go off-line for a certain amount of time after a stimulus has been applied. This waiting period may be termed the refractory period. The refractory period allows the polarization signal to disappear or subside to some minimal amplitude level. Unfortunately, the evoked response often also disappears or subsides during the refractory period. As a result, these pacemakers cannot detect evoked response signals with any degree of confidence. Some pacemakers are designed to come on-line after the refractory period and detect intrinsic signals (e.g., P and R waves). However, information about detected intrinsic signals does not typically provide information about the evoked response signal.
Thus, a need exists in the medical arts for determining reliably whether or not an evoked response signal has occurred in a pacing environment.
Several methods have been proposed in the prior art for improving an implantable device""s ability to detect and measure evoked responses (e.g. captured beats).
For example, U.S. Pat. No. 5,861,013 to Peck et al., entitled xe2x80x9cPeak Tracking Capture Detection Circuit and Methodxe2x80x9d, hereby incorporated by reference in its entirety, discloses detecting an evoked response by noting the polarity of the positive or negative change in voltage with respect to time (or dv/dt) of a waveform incident on the lead electrodes during a period of time immediately after a pacing pulse. An evoked response may reverse the polarity of the polarization signal as detection is occurring; this reversal may be noted. If the magnitude of the polarization signal is so great that the evoked response does not reverse the polarity, an acceleration (increasing magnitude of dv/dt) in the sensed signal or waveform may be noted instead.
U.S. Pat. No. 5,172,690 to Nappholz et al., entitled xe2x80x9cAutomatic Stimulus Artifact Reduction for Accurate Analysis of the Heart""s Stimulated Response,xe2x80x9d hereby incorporated by reference in its entirety, discloses a tri-phasic stimulation waveform consisting of pre-charge, stimulus, and post-charge segments. The duration of the pre-charge segment is varied until the amplitude of the stimulation artifact is small compared to the evoked response.
U.S. Pat. No. 5,431,693 to Schroeppel, entitled xe2x80x9cMethod of Verifying Capture of the Heart by a Pacemaker,xe2x80x9d hereby incorporated by reference in its entirety, discloses a pacemaker that low-pass filters a sensed signal to remove noise and pass frequencies characteristic of the evoked cardiac signal. The filtered signal is processed to render a waveform signal representing the second derivative of the filtered signal. The second derivative filtered signal is further analyzed to detect minimum and maximum amplitude excursions during selected first and second time windows. The amplitude differences measured during the two time windows are compared to one another to determine whether capture has occurred.
U.S. Pat. No. 4,114,627 to Lewyn et al., entitled xe2x80x9cCardiac Pacer System and Method with Capture Verification Signal,xe2x80x9d hereby incorporated by reference in its entirety, discloses a pacer that delivers output stimulating pulses through an output coupling capacitor. During delivery of a stimulating pulse, the sense amplifier is uncoupled from the cardiac electrode. When the stimulating pulse terminates, the output coupling capacitor is coupled to ground through a discharge resistor, thereby discharging electrode polarization.
As discussed above, the most pertinent prior art patents are shown in the following table:
All the patents listed in Table 1 are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, the Detailed Description of the Preferred Embodiments and the claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
The present invention is therefore directed to providing a method and system for discriminating a captured beat from a non-captured beat in cardiac tissue. Such a system of the present invention overcomes the problems, disadvantages and limitations of the prior art described above, and provides a more efficient and accurate means of distinguishing a captured beat.
The present invention has certain objects. That is, various embodiments of the present invention provide solutions to one or more problems existing in the prior art respecting the instantaneous stimulation of a mammalian heart. Those problems include, without limitation: the ability to discriminate between a captured beat and a non-captured beat.
In comparison to known techniques for determining a captured beat, various embodiments of the present invention may provide the following advantage, inter alia, i.e., the accurate distinguishing of a captured beat from a non-captured beat.
Some of the embodiments of the present invention include one or more of the following features: an implantable medical device including at least one sensing lead, at least one pacing lead, a microprocessor and an input/output circuit including a digital controller/timer circuit, an output circuit, a sense amplifier, a peak sense and threshold measurement device, a comparator and an electrogram amplifier.
Furthermore, in accordance with the present invention, an embodiment for a method and system of discriminating a captured beat from a non-captured beat in cardiac tissue is provided. A pulse is transmitted to the cardiac tissue. An evoked response signal is received. The evoked response signal is filtered and analyzed for positive signal components in a predetermined window.